Thermodynamic modeling of atomic bonding in covalent C_x(BN)_1-x alloys

The results of the application of the free energy model (FEM) to the atomic bonding in covalent network C_x(BN)_1-x alloys are presented. The high values of C-C and B-N bond energies strongly favors chemical ordering and phase separation of carbon and boron nitride. Only at temperatures of the order of the melting points are equilibrium bonding configurations including C-B and C-N bonds expected to be present. Two types of structure based on either sp³ (tetrahedral) or sp² (trigonal) configurations with different short and long range ordering are possible in the alloys. The tetrahedral structure will consist of separate clusters of diamond-like carbon and cubic boron nitride. For the hexagonal structure planar bonding causes instead two-dimensional clusters to be present. The weak interlayer bonding makes thermodynamically possible the formation of alternating graphite and BN layers. Special physical properties of the alloys connected with this ordering may be expected.